postgresql - SELECT or INSERT a row in one command

Question

I'm using PostgreSQL 9.0 and I have a table with just an artificial key (auto-incrementing sequence) and another unique key. (Yes, there is a reason for this table. :)) I want to look up an ID by the other key or, if it doesn't exist, insert it:

SELECT id
FROM mytable
WHERE other_key = 'SOMETHING'

Then, if no match:

INSERT INTO mytable (other_key)
VALUES ('SOMETHING')
RETURNING id

The question: is it possible to save a round-trip to the DB by doing both of these in one statement? I can insert the row if it doesn't exist like this:

INSERT INTO mytable (other_key)
SELECT 'SOMETHING'
WHERE NOT EXISTS (SELECT * FROM mytable WHERE other_key = 'SOMETHING')
RETURNING id

... but that doesn't give the ID of an existing row. Any ideas? There is a unique constraint on other_key, if that helps.

Answer 1

Have you tried to union it?

---

Edit - this requires Postgres 9.1:

create table mytable (id serial primary key, other_key varchar not null unique);

WITH new_row AS (
INSERT INTO mytable (other_key)
SELECT 'SOMETHING'
WHERE NOT EXISTS (SELECT * FROM mytable WHERE other_key = 'SOMETHING')
RETURNING *
)
SELECT * FROM new_row
UNION
SELECT * FROM mytable WHERE other_key = 'SOMETHING';

results in:

 id | other_key 
----+-----------
  1 | SOMETHING
(1 row)